Polyglutamine (polyQ) disease is a major cause of inherited neurodegeneration in the United States. The nine known polyQ diseases are caused by CAG repeat expansions that encode glutamine repeats in otherwise unrelated disease proteins. Spinocerebellar ataxia type 3 (SCA3), the most common dominantly inherited ataxia, is caused by an expansion of a glutamine repeat in the C-terminus of ataxin-3. One major question in polyQ disease research is the identity of the putative toxic intermediate(s) that triggers neurodegeneration. This grant aims to identify the toxic species that underlie SCA3. In aim 1, two transgenic SCA3 mouse models will be used to investigate the molecular and cellular pathological features of SCA3 over time in an effort to identify the toxic intermediates that lead to neuronal dysfunction and cell death. Aim 2 addresses the link between mutant (expanded) ataxin-3 and protein quality control mechanisms in disease pathogenesis. Specifically, both cell culture and mouse models will be used to investigate the interactions of ataxin-3 with a known modifier of polyQ disease, the co-chaperone and ubiquitin ligase C-terminus of Hsp70 interacting protein (CHIP). Neurodegeneration in many diseases, including Alzheimer's, Parkinson's and the polyglutamine diseases, is thought to be due to misfolding of proteins in neurons. This project will investigate protein misfolding in polyglutamine disease, which currently affects about 100,000 Americans;however, the findings will be broadly relevant to protein misfolding diseases in general. The results may also identify new therapeutic targets for polyglutamine disease.